1. Field of the Invention
This invention relates to medical diagnostic equipment, and more particularly to blood analyzers for measuring hematocrit value of a blood sample.
2. Description of Related Art
Measurement of hematocrit (i.e., volume of red blood cells per unit volume of whole blood) is useful for monitoring, diagnosing, and otherwise determining the health and status of a medical patient. One method for measuring hematocrit is to directly count the number of red blood cells using a conventional microcentrifuge and modem cell-counting cytometry methods. However, such methods are not amenable to the design of convenient or continuous bedside monitoring instruments. Rather, an indirect method in which blood conductivity (1/R.sub.blood) is measured is preferred for determining hematocrit in order to make it possible to determine hematocrit at bedside. It is well known that the conductivity of a blood sample varies as a function of its hematocrit value. It is also known that other blood components must be taken into account when calculating hematocrit based upon the conductivity of a blood sample. Currently, bridge circuits are used to determine the resistance (R.sub.blood) of a blood sample. A bridge circuit determines resistance by comparing an unknown resistance (i.e., R.sub.blood) with a known resistance.
FIG. 1 is an illustration of a Wheatstone bridge circuit commonly used to determine the resistance (R.sub.blood) of a blood sample. The bridge circuit comprises two fixed resistors R.sub.a, R.sub.b with known values. The values of these resistors R.sub.a, R.sub.b must be known to a very high precision in order to accurately determine the value of the resistance (R.sub.blood) of a blood sample. One terminal of each of the resistors R.sub.a, R.sub.b is coupled together at node 100. A "zero-center" current meter 104 is coupled between the other terminal of each of the known resistors R.sub.a, R.sub.b at nodes 101, 102. Also coupled to the node 102 is a first terminal of the hematocrit sensor. The hematocrit sensor is shown in FIG. 1 as an equivalent resistor having a resistance R.sub.blood. The first terminal of the hematocrit sensor is placed in contact with the blood sample to be measured. A second terminal of the hematocrit sensor which is also in contact with the blood sample is coupled to a node 103. A high precision potentiometer 105 having a resistance R.sub.pot is coupled between nodes 101 and 103 and a voltage source 106 is placed across nodes 100 and 103. It is well known that when no current flows through the current meter 104, the value of the unknown resistance R.sub.blood is equal to: EQU R.sub.pot (R.sub.b /R.sub.a)
However, there are a number of disadvantages to using such a bridge circuit to measure hematocrit. It is typically desirable to have the resistance measurement device interface with a microprocessor or other programmable device in order to allow additional calculations to be performed, such as adjusting the measured resistance to account for electrolytes in the blood, and to allow data to be compiled, logged, and reported. Also, such bridge circuits require adjustments to be made to the potentiometer to set the bridge to null, and calibrations to be made to ensure the accuracy of the bridge circuit (i.e., the accuracy of the known resistances). Furthermore, such bridge circuits require compensation for both temperature and voltage. Still further, the voltage source 106 must be very stable. In addition, such bridge circuits typically require a great deal of space and are expensive due to the number of high precision components required. Therefore, in general, bridge circuits are difficult and expensive to make accurate, inherently instable over long periods of time, and require close tolerance components.
Accordingly, it would be desirable to provide a method and apparatus for measuring the conductivity (or resistance) of a blood sample for use in a hematocrit measurement device which is stable over long periods of time, is inexpensive, can be fabricate in relatively small space, which does not require regular calibration and adjustment, and which can easily interface with a microprocessor or other digital device.